1.Field of the Invention
The present invention relates to an image sensor used in an input section of a facsimile or the like, and more particularly to an improvement of an image sensor having a linear array of a plurality of photodetecting elements each consisting of a photo diode and a blocking diode connected in series and oppositely in polarity.
2.Discussion of the Related Art
Conventionally, an image sensor for reading an image in a facsimile or the like has a linear array 2 of a plurality of photodetecting elements 1 each consisting of a photo diode PD and a blocking diode BD connected in series and oppositely in polarity, as shown in FIG. 6. In the image sensor shown in FIG. 6, the photodetecting element array 2 is segmented into a plurality of groups 3.sub.1 -3.sub.n each consisting of a number of photodetecting elements 1. In reading image information from the photodetecting elements 1, a single shift register SR is matrix-driven to sequentially select the photodetecting element groups 3.sub.1 -3.sub.n and to couple the selected one to a read circuit 4 (see Japanese Patent Unexamined Publication No. Sho. 58-56363).
The image read operation by the image sensor thus constructed will be described.
One of block select switches 7.sub.1 -7.sub.n selects one of the photodetecting element groups 3.sub.1 -3.sub.n, and connects the selected one to a common wire 5. Under this condition, the shift register SR scans the photodetecting elements 1 constituting the selected group, that is, sequentially applies signals to the elements, to charge the reversely biased photo diodes PD. At this time, the blocking diode BD in other groups are reversely biased by power sources 6 that are selected by the block select switches, in order to prevent crosstalk occurrence within the photodetecting element groups.
The above sequence of operations is performed for each photodetecting element group by switching the block select switches 7.sub.1 -7.sub.n. In this way, the photo diodes PD of all the photodetecting elements 1 constituting the photodetecting element array 2, are charged.
During one cycle of the scan, the photo diodes PD are illuminated with light, and emit the charge according to the amounts of light.
The shift register SR sequentially applies read pulses to the photodetecting elements 1 again, to supply charge to the photo diodes PD according to the amounts of discharge. The currents caused by the re-charge are read by the read circuit 4. In this way, the image signals are time-sequentially picked up.
In the image sensor thus constructed and operated, group wires 8.sub.n in groups 3.sub.n other than the group to be read are reversely biased, in order to prevent crosstalk occurrence within the photodetecting element groups 3.sub.n. For this reason, charge is stored in parasitic capacitances 10 between the group wires 8.sub.n and ground. When one of the photodetecting element groups is connected to the common wire 5, the charge stored in the parasitic capacitances flows into the read circuit 4, and acts as noise. To cope with the noise generation problem, it has been proposed that a power source 6', a capacitor 10' having a capacitance value approximately equal to that of the parasitic capacitance 10, and a switch 7', are coupled with the common wire 5. In this construction, when the switch 7' is closed, the charge with the opposite polarity is applied to the group wire 8.sub.n to be read, to cancel the noise.
However, in the above approach, the analog switch used produces switching noise, which reduces the S/N ratio of the detect signal. For example, in a sensor having a resolution of 16 dots/mm, charge generated per pixel is approximately 0.1 pC, while the switching noise of a commercially available CMOS analog switch is 10 pC. Therefore, the valid signal is covered with the switching noise, making it difficult to detect the valid signal.